Two-stage rotary elastic shaft couplings for boat drives have been provided heretofore and reference may be made to the press release of the firm MERCRUISER. This coupling consists basically of a drive-side coupling flange and a driven-side hub connectable to a shaft and axially disposed in a central opening of the coupling flange. A secondary force-transmitting member is connected with the hub and has projections which are circumferentially or angularly spaced but extend parallel to the axis into openings in the coupling flange which are elongated in the circumferential direction.
These projections, which may be pins, function as claws engaging the coupling flange and may be referred to as such. Between the hub and the coupling flange two groups of elastic coupling elements are provided. The first, consisting of an elastic ring, is provided directly between the coupling flange and the hub. The second group of elastic coupling elements are disposed in the elongated openings in the coupling flange and are juxtaposed with the claws to engage them after there has been some relative angular displacement of the coupling flange and the hub.
The first elastic coupling element is softer and more pliable than the second group and the torque transmission at low torques or loads of the coupling and in idling or small partial loads can be effected exclusively by the first elastic coupling body from the coupling flange to the hub. Only with larger loads to full load and higher torque transmission is the second group of the torsionally stiffer elastic bodies automatically brought into play to participate in the torque transmission.
In practice it has been found that this prior art shaft coupling solves a problem which arises with shaft couplings of earlier designs at low loads or at idle, namely, the so-called gear hammering. Such noise arises when the elastic body is not soft or pliable enough. With the two stage MERCRUISER shaft coupling, the coupling flange is connected with the hub by an elastic rubber ring which is relatively pliable. The torque delivered by the flywheel of the boat motor is thus initially transmitted via this elastomeric ring to the hub with which the drive shaft is rotationally coupled. At idle and at low engine speeds, this elastic coupling element alone can serve to drivingly connect the flange with the hub.
The secondary force-transmitting member in this construction is a plate or disk-like member which has the aforementioned claws or pins on which elastomeric sleeves can be provided and which are torsionally stiffer than the elastomeric ring forming the first element. These pins with their sleeves engage in elongated openings of the coupling flange. As long as the drive torque remains in the idle or low-load range, the pins with their elastic sleeves are free to oscillate within the elongated openings and without impacting on the ends or flanks thereof.
With an increase in the driving torque and greater load or resistance at the driven side, the restoring force of the elastic ring is no longer able to maintain the pins free to oscillate and the pins with their sleeves are driven against the trailing flanks defining the respective openings. As a result, therefore, of the greater angular relative displacement of the flange and the hub, the elastomeric sleeves come into contact with edges of the respective openings and thus assume the force and torque-transmitting function.
A two-stage torsional elastic coupling of this type has a damping characteristic which, in the idle and low-load ranges, corresponds effectively only to that of the elastic ring and has a generally flat rise. This curve has a sharp break upward when the second group of torsionally stiffer elastic coupling members, namely the sleeves, comes into play.
This two-stage coupling has been found to substantially reduce the noise in the idle or low-load conditions for a shaft coupling. However, this prior shaft coupling is expensive and of complex construction. Mounting of the elastic ring by bolting or the like with the coupling flange and the hub is time-consuming and vulcanization techniques are even more complex and can render maintenance of the shaft coupling problematical. Mounting and assembly and disassembly are all time-consuming and difficult.